This invention relates to a method of assigning transmission channels to base stations in a cellular mobile radio communication network and, more particularly, in a small cell mobile radio communication network.
Such a cellular mobile radio communication network, or cellular mobile radio network, has a service area divided into a plurality of radio zones or cells. A base station or cell site is situated in each radio zone. The base stations of the cellular mobile radio communication network are in this manner in one-to-one correspondence to the radio zones. At least one mobile station is movable in general in the service area from one radio zone to another radio zone. Each base station is assigned at least one transmission channel for use in communication with a mobile station that is present and alive in the corresponding radio zone. ("Present and alive" as used herein denotes having power and available for transmitting and receiving.) As a consequence, the cellular mobile radio communication network is assigned a plurality of transmission channels.
The cellular mobile radio communication network may be an automobile radio communication network, in which case a mobile station comprises communication terminal equipment installed on an automobile. Alternatively, the cellular mobile radio communication network may be a multi-channel cordless telephone network, in which case a mobile station is a cordless telephone set that may be carried by an attendant.
In the cellular mobile radio communication network, two adjacent ones of the radio zones share an overlapped area. These two adjacent zones will be referred to herein as first and second zones. The base stations corresponding to the first and the second zones will be called the first and second stations. While present and alive in the first zone, a mobile station receives a first down signal from the first station and transmits a first up signal to the first station.
It will be assumed merely for brevity of the description that a particular channel of the transmission channels of the cellular mobile radio communication network is used in carrying the first down and up signals. Under the circumstances, the first up signal reaches the first station as a desired signal through this particular channel.
When the mobile station moves into the overlapped area, the first up signal may still be received at the first station as the desired signal. If the particular channel is also being used by the second station, the first up signal reaches the second station as an interference or undesired signal through the particular channel.
The transmission channels must consequently optimally be assigned to the base stations of the cellular mobile radio communication network. In accordance with fixed channel assignment, the transmission channels are fixedly assigned to the base stations. According to one of various strategies of dynamic channel assignment, each base station measures a carrier-to-interference ratio (CIR) of each of the transmission channels of the network upon occurrence of a channel assignment request or call connection request in order to select one of the transmission channels as a selected or favorite channel if it has a carrier-to-interference ratio above a preselected threshold level, herein called a CIR threshold level.
On carrying out this strategy, it is unnecessary for each of adjacent ones of the base stations and the mobile station or stations currently present and alive in the corresponding radio zones of the base station under consideration and of the adjacent ones of the base stations to measure the carrier-to-interference ratio even when the channel is already used thereby. As a result, this strategy is defective in that the channel under consideration may give rise to interference. Moreover, the mobile station may move into the overlapped area, causing the respective powers of the desired signal and interference signal to vary, and giving rise to similar interference.
Should interference takes place, it is necessary to switch channels. This switch between the channels results in an instantaneous suspension or break in the communication. If no other channels are available, the communication must forcibly be suspended. When such suspensions are frequent, a heavy load is unavoidably imposed on control of the channels.
In order to reduce such an interference probability, a "channel segregation" scheme is revealed in a paper contributed by Yukitsuna Furuya and Yoshihiko Akaiwa to the Second Nordic Seminar on Digital Land Mobile Radio Communication held 14-16 Oct. 1986 in Stockholm, Sweden, and printed in Proceedings, pages 311 to 315. This scheme is disclosed in U.S. Pat. No. 4,747,101 (the '101 patent) issued in Yoshihiko Akaiwa and two others, and assigned to the present assignee. This United States Patent is incorporated herein by reference.
In accordance with the channel segregation scheme, each base station assigns priority degrees or transmission priorities to the transmission channels utilized in the cellular mobile radio communication network, and measures, in connection with each transmission channel, the power which an interference signal has. This is referred to herein as "interference signal power". When the interference signal power is lower or higher than a predetermined level, the base station raises or lowers, respectively, one of the priority degrees into an updated priority degree that is assigned to the transmission channel in question. The base station selects an idle one of the transmission channels as the selected channel and assigns it the highest possible updated priority degree if it has a carrier-to-interference ratio above the preselected threshold level.
Meanwhile, the preselected threshold level may be kept constant throughout the transmission channels. Preferably, in order to further reduce the interference probability, the threshold level is controlled on actually selecting the selected channel.
Another excellent control of channel selection is revealed in Japanese Patent Prepublication (A) No. 104,329 of 1991. In accordance with this control, attention is directed to the fact that the interference probability depends on the velocity or speed of the mobile station. The preselected threshold level is therefore determined dependent on the velocity of the mobile station.
According to the channel segregation scheme wherein the preselected threshold level is kept constant throughout the transmission channels, the interference probability is low for the transmission channel that has a high updated priority degree and is consequently frequently used as the selected channel. Conversely, the interference probability is high for the transmission channel that has a low updated priority degree and is consequently only rarely used as the selected channel. This holds true even if the threshold level is raised in connection with each of the transmission channels utilized in the cellular mobile radio communication network. As a result, the channel segregation scheme involves a problem such that the interference probability is not much reduced. Instead, blocking probability (discussed in detail in the paper cited before) grows high.
When the channel segregation scheme is adopted, the interference probability depends little on the velocity of the mobile station. Instead, it has been found by the present inventor that the interference probability depends on the updated priority degrees. As a consequence, it is not effective under the circumstances to control the preselected threshold level in accordance with the velocity of the mobile station.